1. Field of the Invention
The present invention relates to the field of dynamoelectric machines. It relates to a turbo-generator having a rotor and a stator concentrically surrounding the rotor and separated from the rotor by an air gap, and also having a cooling arrangement in which a main fan draws in a gaseous cooling medium flowing through corresponding cooling passages in the rotor and in the stator and forces it through a cooler back into the cooling passages, the cooling medium flowing through the rotor in the axial direction, issuing in a heated state into the air gap and being returned from the air gap to the main fain.
Publication U.S. Pat. No. 4,379,975, for example, discloses such a turbo-generator.
2. Discussion of Background
To ensure trouble-free operation and to utilize the full output potential, large turbo-generators must be cooled in order to dissipate the heat loss which occurs in the windings and cores of the rotor and stator. A gaseous medium such as air or even hydrogen is usually used for the cooling and is fed through corresponding bores or slits in the rotor and stator and then cooled down again in a cooler. In this case, the winding overhangs of the stator winding, at which an especially large heat loss occurs on account of the conductor geometry and the leakage field associated with it, warrant special attention. If the cooling medium flowing through the rotor and stator is in the process drawn out of the cooling region by main fans sitting on the ends of the rotor and forced into the cooling region again via a downstream cooler, the term "reverse flow cooling" is used.
In the rotor, the cooling medium flows axially from the ends into corresponding bores of the rotor conductor toward the inside, issues radially into the air gap between stator and rotor after absorbing the rotor heat loss, and flows axially in the air gap outward to the main fans again. In this case, the cooling medium issuing from the rotor has a markedly higher temperature than the stator, in particular in the end regions of the rotor as well.
The air gap between rotor and stator is dimensioned to be comparatively narrow, so that a relatively high flow resistance results for the rotor cooling, and this flow resistance hinders the circulation of the cooling medium and thus limits the cooling effect. In addition, the cooling medium heated to a relatively high degree in the rotor gives off heat to the stator when passing or striking the stator bore, as a result of which the cooling of the stator is impaired.
In the publication mentioned at the beginning, it is proposed to attach a short, annular air-gap cylinder piece in the air gap in the end region of the stator, which air-gap cylinder piece, on the rear side, is subjected to a flow of gas from a plurality of radial slits in the stator and, on the front side, prevents the flow of relatively hot gas issuing from the end region of the stator from striking the inner surface of the stator bore or deflects said flow of gas. Although the adverse effect of the cooling gas, heated to an especially high degree, from the winding-overhang region of the rotor on the stator is avoided or reduced by restricting the air-gap cylinder piece to the end regions of the stator, the remaining region of the stator is nonetheless subjected to the relatively hot cooling medium flowing in the air gap, so that the cooling is more likely to be impaired precisely in the stator region close to the axis, where the stator windings are located. In particular, the circulation of the cooling medium through the rotor will be made worse rather than improved by this measure.